Diesel engines, because of the way they operate, emit soot particles or very fine droplets of condensate or a conglomerate of the two (particulates) as well as typical harmful gasoline engine exhausts (i.e., HC and CO). These “particulates” (herein Diesel soot), are rich in condensed, polynuclear hydrocarbons, some of which may be carcinogenic.
As the awareness of the danger Diesel soot presents to health collides with the need for greater fuel efficiency that Diesel engines provide, regulations have been enacted curbing the amount of Diesel soot permitted to be emitted. To meet these challenges, soot filters have been used. When using such a filter, the filter must be periodically regenerated by burning off the soot. This burning of soot results in stresses from axial and radial temperature differences that can cause cracking of the filter.
To overcome stresses ceramic honeycombs such as heat exchangers and filters have reduced the stresses and potential for cracking the honeycombs by assembling smaller honeycombs into larger honeycombs. Cement layers between the honeycombs have been used, for example, to increase the thermal conductivity to reduce the ultimate temperature reached in the assembled honeycomb such as described by EP 1508355. To achieve the improved thermal conductivity, these cements/sealing layers/adhesives have used ceramic particulates to increase the thermal mass/conductivity and ease of application to the smaller honeycomb segments. Often such cements include ceramic fibers, ceramic binder and organic binder such as described by U.S. Pat. No. 5,914,187 to facilitate application of the cement prior to firing (e.g., reduce segregation of particulates) and improve some mechanical property such as toughness of the cement.
Unfortunately, the use of these augmenting materials results in problems in using the cement or reduced effectiveness. For example, the use of organic binder helps in reducing separation of particles in the cement, but then must be removed slowing the process to make the part and also risking damage due to thermal gradients due to combustion of the organics and pressure from evolving gases. The use of fibers also tends to lower the thermal mass and thermal conductivity of the cement layer due to inefficient packing and inability to load the fibers to any great extent into a carrier fluid without excessive viscosity increases.
Therefore, it would be desirable to provide an assembled larger honeycomb from smaller ceramic honeycombs and method to do so that avoids one or more problems described above.